Silicone-type pressure-sensitive adhesive compositions characteristically have better electrical insulation characteristics, a better resistance to heat and cold, and a better tacky adhesiveness for a variety of adherends than do acrylic-type pressure-sensitive adhesive compositions and rubber-type pressure-sensitive adhesive compositions and for this reason are employed for applications such as heat-resistant tapes, electrical insulating tapes, heat-seal tapes, masking tapes for plating applications, masking tapes for heat-treatment applications, and so forth. These silicone-type pressure-sensitive adhesive compositions are categorized in accordance with their curing mechanism into addition reaction-curing grades, condensation reaction-curing grades, and peroxide-curing grades. Among these, the addition reaction-curing grades are well suited for use because they cure rapidly upon standing at room temperature or upon the application of heat and do so without producing by-products.
A silicone-type pressure-sensitive adhesive (1) is disclosed in Patent Reference 1 as an example of an addition reaction-curing silicone-type pressure-sensitive adhesive composition. This silicone-type pressure-sensitive adhesive (1) comprises    (A1) diorganopolysiloxane having alkenyl at both molecular chain terminals (viscosity of at least 500,000 cP),    (B1) organopolysiloxane resin comprising the R3SiO1/2 unit (R in the formula is alkyl, alkenyl, or the hydroxyl group) and SiO4/2 unit,    (C1) organopolysiloxane having at least two silicon-bonded hydrogens per molecule {in a quantity that provides from 1 to 20 silicon-bonded hydrogens in component (C1) per alkenyl in component (A1)},    (D1) a platinum catalyst, and    (E1) organic solvent.
However, such an addition reaction-curing silicone-type pressure-sensitive adhesive exhibits poor tackiness for silicone rubber and readily comes unstuck therefrom.
With the intention of improving the tacky adhesion for silicone rubber, a composition (1) is disclosed in Patent Reference 2, which comprising    (A) chain-form organopolysiloxane that has at least two vinyl groups in terminal position and/or pendant position, and/or chain-form organopolysiloxane that has silanol in terminal position and that may or may not have vinyl in pendant position,    (B) silanol-functional organopolysiloxane comprising the triorganosiloxy unit and SiO2 unit wherein the former-to-latter molar ratio is in the range from 0.5 to 1.2,    (C) organohydrogenpolysiloxane that contains at least two silicon-bonded hydrogen atoms,    (D) an organosilicon compound that contains the B—O—Si bond, and    (E) a platinum catalyst.And a composition (2) is provided in Patent Reference 2, which comprising chain-form organopolysiloxane having silanol in terminal position, component (B), component (D), and organoperoxide.
These silicone-type pressure-sensitive adhesives characteristically comprise an organosilicon compound that has the B—O—Si bond in each molecule and provides an improved tackiness for silicone rubber. However, the organosilicon compound is the reaction product of boric acid with a silane or siloxane, and few such compounds are commercially available or easy to be acquired. As a consequence, this has led to demand for a silicone-type pressure-sensitive adhesive that is suitable for mass production at an industrial level and that provides additional improvements in the tacky adhesion for silicone rubber.
On the other hand, the silicone-type pressure-sensitive adhesive composition (3) provided in Patent Reference 3 comprises    (A) organopolysiloxane yielded by the partial condensation of            (a) diorganopolysiloxane gum in which 10 to 80 mol % of the molecular chain end groups are alkenyl and the remainder is silicon-bonded hydroxyl groups        with        (b) organopolysiloxane that has an average of at least one silicon-bonded hydroxyl group or alkoxyl group in the molecule and that is represented by the average unit formula (R13SiO1/2)x(SiO4/2)1.0 (R1 is hydrocarbyl group and x is 0.5 to 1.0)        under the effect of        (c) an amine compound that has a boiling point at ambient pressure of −10 to 200° C.;            (B) organopolysiloxane that has an average of at least two silicon-bonded hydrogens in each molecule; and    (C) a platinum catalyst.
However, the silicone-type pressure-sensitive adhesive described in Patent Reference 3 does not exhibit a sufficient adhesive strength for silicone rubber. Moreover, when a pressure-sensitive tape or tacky tape having a silicone-type pressure-sensitive adhesive layer yielded by the cure of this adhesive on a tape is exposed to high temperatures while applied on an adherend, a sticky residue is produced when this pressure-sensitive adhesive tape or tacky tape is then peeled off.
Organotitanium compounds, on the other hand, are known as curing catalysts for room temperature-curable silicone rubber compositions (for example, Patent References 4 to 6) and are also easily acquired on an industrial basis. These organotitanium compounds are exemplified by tetraalkoxytitanium, titanium complexes comprising titanium-acetylacetonate ligand, and titanium complexes comprising titanium-alkyl acetylacetate ligand.
Organotitanium compounds are also known as adhesion promoters for addition reaction-curing silicone rubber compositions that contain silica and are added with the goal of improving the adhesive strength for a variety of substrates (for example, Patent References 7 to 11). While the adhesive strength is increased when these organotitanium compounds are used as an additive for an addition reaction-curing silicone-type pressure-sensitive adhesive that will be converted into a thin layer, a sticky residue is produced on the adhered surface when an adhesive layer comprising such a composition is bonded with an adherend, exposed to high temperatures, and then peeled off.
In view of the preceding, there has been some demand for a silicone-type pressure-sensitive adhesive that, through its disposition as a pressure-sensitive adhesive layer on an organic resin that has been separately cured from a target adherend, can as necessary bring about strong adhesion by the organic resin to silicone rubber, and that even after application of the adherends and exposure to high temperatures enables, as the need arises, facile peeling with the production of almost no sticky residue on either adherend.